Incorporation of radioactive amino acids into proteins of the microsome fraction of guinea-pig liver in a cell-free system

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Abstract
A study was made of the incorporation of C14-amino acids into microsome proteins in a cell-free system prepared from guinea-pig liver. The cell-free system contained microsome and cell-sap fractions and was incubated for varying periods of time with a mixture of C14- amino acids. The microsome material was then isolated and fractionated by the extraction procedure of Simkin and Work (1957). C14- amino acids were incorporated into the proteins of the microsome subfractions at quite different rates. The protein of a subtraction (B) which contained ribonucleic acid (RNA) attained a specific activity more than twice that of the whole microsome protein, whereas the activity of the protein of another RNA-containing subfraction (C) increased more slowly. Comparisons with previous application of the same fractionation technique to guinea-pig liver microsome material labeled in vivo showed a pattern of incorporation into subfractions which was quite different, particularly with respect to the behavior of the proteins associated with RNA. Incubation produced an appreciable loss of RNA in the microsome material. The significance of the incorporation of C14-amino acids into micro-some protein in this cell-free system is discussed. A brief study was also made of the changes in the specific activity of both the soluble protein and of the trichloroacetic acid-soluble material of the cell-sap fraction after incubation of this cell-free system with C14-amino acids.